Graphites are made up of layered planes of hexagonal arrays or networks of carbon atoms. These layered planes of hexagonally arranged carbon atoms are substantially flat, covalently bonded in the flat layered planes, and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size; the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess transversely isotropic structures and flexible graphite produced thus exhibit or possess many anisotropic properties that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated structures of carbon that are structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or direction. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or direction may be considered as the directions parallel to the carbon layers or the direction perpendicular to the “c” direction. The natural graphites suitable for manufacturing flexible graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are weak van der Waals forces as compared to the covalent bonds in the layered planes. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
With respect to the above treating of natural graphite, such as natural graphite flake, with an intercalant, e.g., a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite may be hereafter referred to as “particles of intercalated graphite”. Upon exposure to high temperature, the particles of intercalated graphite expand in dimension as much as 80 or more times its original volume in an accordion-like fashion in the “c” direction, i.e., in the direction perpendicular to the crystalline planes of the graphite. The exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets which, unlike the original graphite flakes, can be formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact.
A common method for manufacturing graphite sheet, e.g., foil from flexible graphite, is described by Shane, et al. in U.S. Pat. No. 3,404,061 the disclosure of which is incorporated herein by reference. As shown in FIG. 1, in the typical practice of the Shane, et al. method, natural graphite flakes 12 are intercalated 14 by dispersing the flakes in a solution containing a mixture of nitric and sulfuric acid. The intercalation solution may contain acidic compounds and other intercalating agents known in the art. Examples of acidic compounds include solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g., trifluoroacetic acid.
After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed. The quantity of intercalation solution retained on the flakes after draining may range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 50 to 120 pph. Alternatively, the quantity of the intercalation solution may be limited to between 10 to 50 parts of solution per hundred parts of graphite by weight (pph) which permits the washing step to be eliminated as taught and described in U.S. Pat. No. 4,895,713 the disclosure of which is also herein incorporated by reference. Upon exposure 16 to high temperature, e.g., 700° C. to 1000° C., the particles of intercalated graphite expand as much as 80 to 1000 times its original volume in an accordion-like fashion in the c-direction, i.e., in the direction perpendicular to the crystalline planes of the constituent graphite particles. As previously stated, the expanded graphite 18 is vermiform in appearance, and is therefore commonly referred to as worms. The worms may be compressed together into flexible sheets which, unlike the original graphite flakes, can be formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact as hereinafter described.
A drawback of the above intercalation process is that it requires significant remediation of effluents generated during the process. The process produces various species of sulfuric and nitrous compounds in both liquid and gas phases that require remediation. There is a need to develop an intercalation process that will reduce, preferably eliminate, the production of the environmentally unfriendly sulfuric and/or nitric species and likewise reduce, preferably eliminate, the use of chemical compounds to treat the environmentally unfriendly sulfuric and/or nitric species.
Another drawback of the intercalation and exfoliation process is that the above process cannot be used to control the amount of exfoliation or to produce nano-particle sized graphite flakes. Consequently, the expanded graphite flake produced from the process will have a thickness of at least 10 microns or greater, typically at least 50 microns or more. Therefore, a need also exists to be able to control the amount of exfoliation and to be able to produce nano-sized particles of graphite.